Conventionally a technique of measuring a height or an altitude (height in vertical direction above sea level 0 m) using a barometric altimeter is known. This technique for measurement is used in some cases on earth as in the measurement of the current height during mountain climbing, or is used in other cases high in the sky that is in the atmosphere away from the earth as in the measurement of the height of an airplane.
Such a barometric altimeter measures the height based on the following principle.
That is, air present at some part on earth receives force corresponding to the weight of air above the part in the sky from the vertically above (from the sky) of the air due to the attractive force (gravity) of the earth. This means that the amount or the density of air is always more (larger) on the surface of the earth than in the sky, resulting in that the pressure of air, i.e., the barometric pressure also is always higher on the surface of the earth than in the sky. Assuming that air is ideal gas, it is well known as the theory of hydrodynamics that the barometric pressure decreases in an exponential manner as it moves up in the sky (with an increase in height).
The barometric altimeter often measures barometric pressure using a sensor generally called an absolute pressure sensor. This absolute pressure sensor is capable of measuring the barometric pressure in the atmosphere with reference to vacuum (0 barometric pressure or 0 hectopascal). As is commonly said as a high pressure system and a low pressure system, a change in barometric pressure is based on a change in weather, or conversely a change in barometric pressure can be an important index to forecast a change in weather. The absolute pressure sensor measures the barometric pressure as the index, and a similar sensor is used for the barometric altimeter as well.
As stated above, the barometric altimeter is configured to find the height or the altitude by measuring the barometric pressure using an absolute pressure sensor or the like. That is, in order to find the height or the altitude from the barometric pressure measured by the absolute pressure sensor or the like, a certain functional relationship between the barometric pressure and the height has to be found. For the practical operation, a qualitative relationship of just a decrease in an exponential manner as in the above-mentioned ideal gas is not sufficient, and another functional relationship that is more quantitative has to be found, for example.
A technique for such a functional relationship is disclosed in Patent Literature 1. This technique is to select a reference temperature T0 at the sea level 0 m based on the climate zones (temperate, tropical and polar) and the date in the following relational expression specified by the international standard indicating the relationship among the height, the barometric pressure and the temperature, and to find the height H from the barometric pressure P0 at the sea level 0 m and the barometric pressure P at the measurement position:H=15385×T0×[1−(P/P0)0.190255].